Production method for improving impact toughness stability of thick high-strength steel Q550D

Abstract

The invention discloses a production method for improving the impact toughness stability of thick-specification high-strength steel Q550D. The thick-specification high-strength steel Q550D comprises the following chemical components in percentage by weight: 0.15-0.17% of C; 0.23 to 0.33 percent of Si; mn: 1.30 to 1.50%; 0.02 to 0.03 percent of Nb; 0.01 to 0.02 percent of Ti; cr: 0.38 to 0.48%; less than or equal to 0.015% of P; s < = 0.005%; b: 0.0008% to 0.0015% of the total weight of the The content of Als is 0.030 to 0.040 percent; o: < = 0.0035%; n: less than or equal to 0.0050%; the production method comprises the steps of KR desulfurization slagging-off, converter smelting, LF refining, RH refining, continuous casting, heating, rolling and heat treatment. The invention aims to meet the depth requirement of the through quenching layer of the thick steel plate, ensure the strength performance of the high-strength steel Q550D and avoid great fluctuation of low-temperature toughness caused by the boron element.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for medium and thick plates, and particularly relates to a production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D. Background Technology

[0002] Thick-gauge high-strength steel Q550D is mainly used in coal mining machinery and engineering machinery, such as hydraulic supports, port cranes, and flatbed trucks, all of which require high strength and good low-temperature impact toughness. Therefore, in recent years, many researchers have devoted themselves to improving the strength and toughness of thick-gauge high-strength steel, enhancing its impact toughness while ensuring high strength, thereby obtaining high-strength steel plates with better overall mechanical properties.

[0003] Currently, most steel companies use an offline quenching + high-temperature tempering process to produce thick-gauge high-strength steel Q550D. However, some companies are limited by the capacity of their quenching equipment, resulting in limited hardenability of thick steel plates. They need to compensate for this limitation by increasing the amount of hardenable elements in the chemical composition design. Considering both production costs and impact toughness, producing 60-80mm thick Q550D high-strength steel on a wide and thick plate production line is quite challenging and requires a high level of technical control.

[0004] Patent CN 117721281 A, entitled "A Method for Improving the Low-Temperature Impact Toughness of Online Quenched High-Strength Steel," provides a production method for producing Q550D and Q690D high-strength steel using a lift-line quenching process. While this method employs online quenching technology, it has significant limitations regarding steel plate thickness. This patent utilizes offline quenching combined with tempering and weak cooling techniques to ensure the uniformity of performance in thicker steel plates.

[0005] Patent CN 103556078 B, entitled "A Production Method for High-Strength Q550D Extra-Thick Steel Plate with Quenching and Tempering," provides an offline quenching and tempering process. This method uses a quenching temperature of 920–930℃ and a tempering temperature of 660–680℃. This patent uses a quenching temperature of 910±10℃ and a tempering temperature of 540±20℃. Furthermore, this patent emphasizes the precise control of the boron element, which increases the stability of impact performance while ensuring tensile strength.

[0006] The patent CN 103468903 B, entitled "Method for Improving the Low-Temperature Impact Toughness of High-Strength Steel," provides a production process for thick-gauge high-strength steel using TMCP followed by tempering. This method involves controlled rolling followed by controlled cooling, and then direct tempering at 650–680℃. Through precise control of the hardenability element "boron," combined with subsequent heat treatment and tempering processes, this patent places lower demands on rolling mill equipment and results in lower alloy costs. Summary of the Invention

[0007] The purpose of this invention is to provide a production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D. Based on existing alloy composition design, through refined control of the hardenability element boron, and in conjunction with subsequent heat treatment and tempering processes, the traditional air cooling after tempering is changed to water spray cooling, enabling rapid cooling of the steel plate and effectively avoiding second-type temper brittleness. This satisfies the hardenability depth requirements for thick-gauge steel plates, effectively ensuring the strength performance of high-strength steel Q550D, while avoiding significant fluctuations in low-temperature toughness caused by boron.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0009] This invention discloses a production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D. The chemical composition of the high-strength steel, by weight percentage, includes: C: 0.15–0.17%; Si: 0.23–0.33%; Mn: 1.30–1.50%; Nb: 0.02–0.03%; Ti: 0.01–0.02%; Cr: 0.38–0.48%; P≤0.015%; S≤0.005%; B: 0.0008–0.0015%; Als: 0.030–0.040%; O: ≤0.0035%; N: ≤0.0050%; the remainder being iron and unavoidable impurities. The production method includes:

[0010] 1) KR desulfurization and converter steelmaking: After hot metal pretreatment, the sulfur content of the hot metal is S≤0.010%, and the temperature is ≥1280℃. Before the hot metal enters the converter, the slag is removed. The converter endpoint control CT coordinates the tapping of steel, P≤0.010%, S≤0.015%, and the tapping time is 4~7min. Add aluminum and iron when 1 / 5 of the steel is tapped; add aluminum and iron to deoxidize the molten steel. Alloys other than Al are added according to normal requirements. Add the alloys when 2 / 5 of the steel is tapped. During the tapping process, add appropriate amounts of modifier and lime according to the endpoint oxygen content.

[0011] 2) Ladle refining: The LF furnace refining uses oxide metallurgy technology to slag and deoxidize the molten steel, quickly producing white slag and ensuring that the white slag time is more than 15 minutes to stabilize the slag basicity; before leaving the LF furnace, the alloy is added to the target range as much as possible to ensure that the S content is less than 0.003%, and B-Fe=0.018~0.020 is added after feeding wire and blowing argon for 5 minutes;

[0012] 3) RH refining: Try not to adjust the composition of RH. All composition adjustments should be completed in LF. RH vacuum treatment for 15-17 minutes, RH deep vacuum circulation time should be more than 15 minutes, and argon soft blowing time should be more than 10 minutes.

[0013] 4) Continuous casting: Strengthen the airtightness between the ladle, tundish, and crystallizer, and ensure the thickness of the tundish liquid level covering agent. Strengthen the tundish argon purging to avoid gas intake during continuous casting. The nitrogen increase is controlled within 5 ppm throughout the process. The liquidus temperature is 1520℃, the tundish superheat is controlled at 23~33℃, the continuous casting speed adopts the production speed of the tundish billet and implements steady-state casting. The thickness of the continuously cast billet is 250mm. Protective casting is implemented throughout the continuous casting process.

[0014] 5) Heating: The heating temperature is 1220~1260℃, and the core temperature of the billet after exiting the furnace is greater than 1180℃. Ensure that the heat soaking zone is held for ≥40 minutes to ensure that the billet is thoroughly and evenly heated.

[0015] 6) Rolling process: Two-stage controlled rolling. The first stage starts at a rolling temperature greater than 1100 ℃ with a reduction rate of 60% to 70%. The second stage starts at a rolling temperature of 880 to 910 ℃ with a reduction rate of ≥12% for each pass and a cumulative reduction rate of ≥40% for the last three passes. Water cooling after rolling: final cooling temperature of 620 to 680 ℃, slow cooling by stacking.

[0016] 7) Heat treatment process: Quenching + tempering. The quenching temperature is 910±10℃, and the furnace time is calculated as t=1.4min / mm×h (thickness) mm + holding time. After quenching, air cool to room temperature. The tempering temperature is 540±20℃, and the furnace time is calculated as t=2.6min / mm×thickness h mm + holding time. After tempering, high-pressure water is sprayed on the surface of the steel plate, and the steel plate is taken out of the furnace when the temperature drops below 300℃.

[0017] Furthermore, the chemical composition of the high-strength steel by weight percentage includes: C 0.157%, Si 0.26%, Mn 1.40%, P 0.011%, S 0.002%, Als 0.033%, Nb 0.022%, Ti 0.013%, Cr 0.412%, B 0.0012%, O 0.0032%, N 0.0040%; the balance being Fe and unavoidable impurities.

[0018] Furthermore, the chemical composition of the high-strength steel by weight percentage includes: C 0.158%, Si 0.26%, Mn 1.41%, P 0.012%, S 0.002%, Als 0.034%, Nb 0.023%, Ti 0.016%, Cr 0.410%, B 0.0010%, O 0.0030%, N 0.0043%; the balance being Fe and unavoidable impurities.

[0019] Furthermore, the chemical composition of the high-strength steel by weight percentage includes: C 0.166%, Si 0.27%, Mn 1.39%, P 0.014%, S 0.003%, Als 0.033%, Nb 0.024%, Ti 0.015%, Cr 0.410%, B 0.0011%, O 0.0028%, N 0.0041%; the balance being Fe and unavoidable impurities.

[0020] Furthermore, the finished product thickness is 60-80mm.

[0021] Furthermore, the quenching temperature is 909-912℃, and the holding time is 20min.

[0022] Furthermore, the tempering temperature is 540-544℃, and the holding time is 40 minutes.

[0023] The steel plate thickness described in this invention is 60mm to 80mm. Since the high-strength steel with a yield strength of 550MPa contains a certain amount of microalloying elements such as Nb, Cr, and B, a higher heating temperature is used for the slab. This ensures that Nb, Cr, and B can fully dissolve during heating, allowing the carbon and nitrides of the microalloying elements to fully precipitate during subsequent rolling and cooling, thus improving the microstructure of the steel plate. The higher heating temperature also makes it easier to remove the iron oxide scale formed during the heating process, which is beneficial for controlling the surface quality of the steel plate. The longer soaking time ensures that the slab exits the furnace at a more uniform temperature, resulting in a more uniform steel plate structure and properties. The rolling stage employs a low-speed, high-reduction rolling strategy. The large single-pass reduction rate allows the rolling deformation to fully penetrate to the center of the steel plate, effectively refining the austenite grains and homogenizing the austenite microstructure. Simultaneously, the high-temperature welding effect generated during rolling largely eliminates defects such as porosity and microcracks within the cast billet, increasing the steel plate's density and improving its overall material properties, thus providing favorable conditions for the heat treatment process. The subsequent quenching and tempering process strengthens the austenite grain boundaries in the tempered state with alloying elements, preventing transgranular fracture caused by impact at low temperatures and effectively improving the steel's impact toughness. Furthermore, after tempering, high-pressure water spraying rapidly lowers the steel plate temperature. This refines the austenite grains and makes the grain boundaries serrated, increasing the grain boundary area and reducing contamination of the grain boundaries by impurities such as carbon, nitrogen, and phosphorus during tempering, thereby mitigating type II temper embrittlement.

[0024] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0025] This invention employs a low-composition design, using inexpensive alloy boron to improve the hardenability of thick-gauge high-strength steel Q550D, ensuring the strength performance of the steel plate. At the same time, the amount of alloy boron added is strictly limited and controlled within the range of 0.0008 to 0.0015%, so as to reasonably regulate the balance between the strength and toughness of the steel plate.

[0026] Through strict control of smelting, continuous casting, heating, and rolling processes, favorable conditions are created for subsequent quenching and tempering processes. The heat treatment adopts a tempering weak cooling technology, which effectively reduces the ductile-brittle transition temperature of the steel plate, suppresses the second type of tempering brittleness, improves the low-temperature impact toughness of the steel plate, increases the performance stability of high-strength steel plates, and significantly improves the performance qualification rate. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings.

[0028] Figure 1 This is a metallographic photograph of the product after quenching and tempering in Embodiment 1 of the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to embodiments.

[0030] Example 1

[0031] The slab to be rolled after smelting and continuous casting is placed in a heating furnace and heated for 240 minutes, followed by a soaking time of 35 minutes. The chemical composition of the slab is as follows (mass percentage): C 0.157%, Si 0.26%, Mn 1.40%, P 0.011%, S 0.002%, Al 0.033%, Nb 0.022%, Ti 0.013%, Cr 0.412%, B 0.0012%, O 0.0032%, N 0.0040%; the balance being Fe and unavoidable impurities. The slab is rolled into a 60mm thick steel plate. Detailed rolling process is shown in Table 1, heat treatment process in Table 2, and mechanical properties in Table 3.

[0032] Example 2

[0033] The slab to be rolled after smelting and continuous casting was placed in a heating furnace and heated for 230 minutes, followed by a soaking time of 33 minutes. The chemical composition of the slab, by mass percentage, was: C 0.158%, Si 0.26%, Mn 1.41%, P 0.012%, S 0.002%, Als 0.034%, Nb 0.023%, Ti 0.016%, Cr 0.410%, B 0.0010%, O 0.0030%, N 0.0043%; the balance being Fe and unavoidable impurities. The slab was rolled into a 70mm thick steel plate. Detailed rolling processes are shown in Table 1, heat treatment processes in Table 2, and mechanical properties in Table 3.

[0034] Example 3

[0035] The slab to be rolled after smelting and continuous casting is placed in a heating furnace and heated for 260 minutes, followed by a soaking time of 40 minutes. The chemical composition of the slab is as follows (mass percentage): C 0.166%, Si 0.27%, Mn 1.39%, P 0.014%, S 0.003%, Al 0.033%, Nb 0.024%, Ti 0.015%, Cr 0.410%, B 0.0011%, O 0.0028%, N 0.0041%; the balance being Fe and unavoidable impurities. The slab is rolled into a steel plate with a thickness of 80 mm. Detailed rolling processes are shown in Table 1, heat treatment processes in Table 2, and mechanical properties in Table 3.

[0036] Table 1 Rolling process parameters for Examples 1-3

[0037]

[0038] Table 2 Heat treatment process parameters for Examples 1-3

[0039]

[0040] Table 3 Mechanical properties of Examples 1-3

[0041]

[0042] like Figure 1 As shown: From the microstructure after quenching and tempering, it can be seen that the microstructure at 1 / 4 of the steel plate thickness is bainite and pearlite with a small amount of ferrite.

[0043] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D, characterized in that, The chemical composition of the high-strength steel, by weight percentage, includes: C: 0.15–0.17%; Si: 0.23–0.33%; Mn: 1.30–1.50%; Nb: 0.02–0.03%; Ti: 0.01–0.02%; Cr: 0.38–0.48%; P≤0.015%; S≤0.005%; B: 0.0008–0.0015%; Als: 0.030–0.040%; O: ≤0.0035%; N: ≤0.0050%; the remainder being iron and unavoidable impurities; its production method includes: 1) KR desulfurization and converter steelmaking: After hot metal pretreatment, the sulfur content of the hot metal is S≤0.010%, and the temperature is ≥1280℃; before the hot metal enters the converter, the slag is removed, and the converter endpoint control CT coordinates the tapping of steel, P≤0.010%, S≤0.015%, and the tapping time is 4~7min, with the addition of aluminum and iron when 1 / 5 of the steel is tapped; aluminum and iron are added to deoxidize the molten steel, and alloys other than Al are added according to normal requirements, with the alloys added when 2 / 5 of the steel is tapped; during the tapping process, appropriate amounts of modifier and lime are added according to the endpoint oxygen content; 2) Ladle refining: The LF furnace refining uses oxide metallurgy technology to slag and deoxidize the molten steel, quickly producing white slag and ensuring that the white slag time is more than 15 minutes to stabilize the slag basicity; before leaving the LF furnace, the alloy is added to the target range as much as possible to ensure that the S content is less than 0.003%, and B-Fe=0.018~0.020 is added after feeding wire and blowing argon for 5 minutes; 3) RH refining: Try not to adjust the composition of RH. All composition adjustments should be completed in LF. RH vacuum treatment for 15-17 minutes, RH deep vacuum circulation time should be more than 15 minutes, and argon soft blowing time should be more than 10 minutes. 4) Continuous casting: Strengthen the airtightness between the ladle, tundish, and crystallizer, and ensure the thickness of the tundish liquid level covering agent. Strengthen the tundish argon purging to avoid gas intake during continuous casting. The nitrogen increase is controlled within 5 ppm throughout the process. The liquidus temperature is 1520℃, the tundish superheat is controlled at 23~33℃, the continuous casting speed adopts the production speed of the tundish billet and implements steady-state casting. The thickness of the continuously cast billet is 250mm. Protective casting is implemented throughout the continuous casting process. 5) Heating: The heating temperature is 1220~1260℃, and the core temperature of the billet after exiting the furnace is greater than 1180℃. Ensure that the heat soaking zone is held for ≥40 minutes to ensure that the billet is thoroughly and evenly heated. 6) Rolling process: Two-stage controlled rolling. The first stage starts at a rolling temperature greater than 1100 ℃ with a reduction rate of 60% to 70%. The second stage starts at a rolling temperature of 880 to 910 ℃ with a reduction rate of ≥12% for each pass and a cumulative reduction rate of ≥40% for the last three passes. Water cooling after rolling: final cooling temperature of 620 to 680 ℃, slow cooling by stacking. 7) Heat treatment process: Quenching + tempering. The quenching temperature is 910±10℃, and the furnace time is calculated as t=1.4min / mm×h+holding time. After quenching, air cool to room temperature. The tempering temperature is 540±20℃, and the furnace time is calculated as t=2.6min / mm×h+holding time. Where h is the thickness of the steel plate in mm. After tempering, high-pressure water is sprayed on the surface of the steel plate, and the steel plate is removed from the furnace when the temperature drops below 300℃.

2. The production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D according to claim 1, characterized in that, The chemical composition of the high-strength steel, by weight percentage, includes: C 0.157%, Si 0.26%, Mn 1.40%, P 0.011%, S 0.002%, Als 0.033%, Nb 0.022%, Ti 0.013%, Cr 0.412%, B 0.0012%, O 0.0032%, N 0.0040%; the balance being Fe and unavoidable impurities.

3. The production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D according to claim 1, characterized in that, The chemical composition of the high-strength steel by weight percentage includes: C 0.158%, Si 0.26%, Mn 1.41%, P 0.012%, S 0.002%, Als 0.034%, Nb 0.023%, Ti 0.016%, Cr 0.410%, B 0.0010%, O 0.0030%, N 0.0043%; the balance being Fe and unavoidable impurities.

4. The production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D according to claim 1, characterized in that, The chemical composition of the high-strength steel by weight percentage includes: C 0.166%, Si 0.27%, Mn 1.39%, P 0.014%, S 0.003%, Als 0.033%, Nb 0.024%, Ti 0.015%, Cr 0.410%, B 0.0011%, O 0.0028%, N 0.0041%; the balance being Fe and unavoidable impurities.

5. The production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D according to claim 1, characterized in that, The finished product thickness is 60-80mm.

6. The production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D according to claim 1, characterized in that, Quenching temperature 909-912℃, holding time 20min.

7. The production method for improving the impact toughness stability of thick-gauge high-strength steel Q550D according to claim 1, characterized in that, Tempering temperature 540-544℃, holding time 40min.

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